In the first study in humans of the selective serotonin receptor agonist, m-chlorophenylpiperazine, significant neuroendocrine, behavioral and temperature changes were observed to follow oral administration of 0.5 mg/kg of the drug in 14 normal volunteers. Nocturnal elevations in N-acetyl-serotonin (but not in serotonin) were found to be associated with the diurnal melatonin rhythm in rhesus monkey cerebrospinal fluid. Evidence that dopamine and L-dopa may produce their cardiovascular effects in humans via the production of norepinephrine and epinephrine was obtained; in contrast, dopamine agonists such as bromocriptine reduced plasma human norepinephrine concentrations. Physiologic factors controlling the release of atrial natriuretic peptide (ANP) from rodent heart were evaluated using a newly-developed radioimmunoassay. Basal levels of this peptide in conscious rats were found to be considerably lower than those previously reported in rats studied under anesthesia. Fluid volume loading using saline or glucose increased plasma ANP concentrations 4-5 fold; similar effects did not occur in rats studied during halothane anesthesia or in pitched rats, raising the possibility of CNS neural or hormonal influence on ANP release.